1. Field of the Invention
The present invention relates generally to furnaces, and more particularly; to a combustion control apparatus for a powdered coal burning furnace which monitors the amounts of noxious substances contained within the burning waste gases, and the amounts of unburnt substances within the ashes, as well as the power data of a pulverizing mill so as to operate the combustion furnace safely and efficiently.
2. Description of the Prior Art
In recent years, with coal having attained a position as a viable alternative energy source with respect to oil, powdered coal burning technology for generator boilers is attracting attention. The technology itself is already an established one, in which the coal is pulverized within a pulverizing mill and the powdered coal, which is separated from coarse grains of coal by means of a fine/coarse grain separator, is injected in the form of a gas from a burner into a furnace for combustion.
FIG. 3 shows a schematic configuration of a generator boiler using a powdered coal combustion system. In the Figure, the coal deposited within a charging mechanism 10 is fed to a pulverizing mill 11 where it is pulverized by means of rollers 12 into small grains which are separated by means of a fine/coarse grain separator 13 into coarse grains and fine grains of coal. Two types of fine/coarse grain separators are available: one is a vane type that separates fine grains from coarse grains by changing the angle of the vanes and the other is a rotary type that utilizes centrifugal force for separating the fine grains from the coarse grains of coal.
The powdered fine grains of coal extracted by means of the fine/coarse separator 13 are fed together with primary air to a burner 15 of a furnace 14. The primary air serves two purposes-drying the powdered coal so as to make it easier to burn and carrying the powdered coal to the burner. The primary air accounts for 10-30 percent of the amount of air required for combustion. The remainder of the air is supplied as secondary air within the vicinity of the nozzle of the burner 15. Tertiary air may be supplied to ensure stable ignition or adjust the shape of the flame. From an appropriate position within the furnace 14 remote from the burner 15, air for a second-stage combustion (in connection with a two-stage combustion method) is supplied in the direction of the propagation of burning gas.
The two-stage combustion method supplies combustion air in two stages into the furnace 14. That is, the first-stage air (primary to tertiary air) from the burner 15 is intentionally undersupplied so as to cause incomplete combustion and produce a reducing atmosphere in order to suppress generation of nitrogen monoxide (NO), and the second-stage air (for second-stage combustion) is supplied from an appropriate location remote from the burner 15 so as to make up for the air deficiency in order to burn the fuel completely. The first and second air stages are fed from a delivery air blower 16 through an air preheater 17, with the amount of second-stage combustion air adjusted by means of a second-stage air damper 18.
Heat generated by means of the furnace 14 is transmitted to water passing through an evaporator tube 19 by means of radiation or through contact with gases, thereby evaporating the water. The burning gas is passed through the air preheater 17 where the heat of the burning gas is collected, and is then discharged by means of a suction air blower 20 from a stack 21.
In operation of the boiler, it is necessary to minimize the amount of noxious emissions from the burning gases such as, for example, nitrogen oxides NO.sub.x and sulfur oxides SO.sub.x to a level which is within an allowable range while at the same time improving the combustion efficiency. It is especially noted that with those boilers using coal as a fuel, the rate of combustion is far slower than those boilers or furnaces which use oil and gas as their fuels, and therefore the temperature of the furnace tends to be reduced, which in turn increases the amount of unburned substances (H.sub.2, CH.sub.4, and the like) present within the ash that affect the combustion efficiency. Furthermore, since the nitrogen components contained within the coal itself convert into nitrogen oxides NO.sub.x during combustion, the aforenoted combustion process contributes to a significant increase in nitrogen oxides NO.sub.x when compared with oil and gas fuels and their corresponding furnaces.
Therefore, during the operation of such boilers, sensors need to be installed at the outlet or within the flue of the furnace 14 so as to monitor the components of the exhaust gases. Any increase in the amount of unburned substances within the ash should be dealt with by reducing the grain size of the powdered coal by controlling the fine/coarse grain separator 13 so as to increase the combustion efficiency. In order to cope with an increase in the amount of nitrogen oxides NO.sub.x, the two-stage combustion air ratio needs to be changed so as to lower the nitrogen oxides NO.sub.x emissions below the limit.
The amount of unburned substances remaining within the ash varies greatly depending upon the size of the coal grains burned within the burner 15. The finer the grain size, the greater the surface area by means of which the coal grains contact the air for combustion and the smaller the amount of unburned components within the ash. The nitrogen oxides NO.sub.x density also varies according to the grain size and kind of coal. On the other hand, the two-stage burning method for reducing the nitrogen oxides NO.sub.x emissions increases the amount of unburned substances since it lowers the in-furnace temperature. The control of the fine/coarse grain separator 13 that determines the grain size is subject to limitations imposed by means of the operating power of the pulverizing mill, which in turn varies according to the kind and amount of coal supplied and also according to the roller friction conditions.
In this way, the plant status characteristics including NO.sub.x the nitrogen oxides density, the unburned components within the ash the pulverizing mill power conditions, the two-stage combustion air ratio, and the control quantities of the fine/coarse grain separator all interfere with each other. Therefore, the optimum operation of the plant has currently or conventional required the skill and experience of a veteran operator.